Typically, an exhaust gas sensor may be positioned in an exhaust system of a vehicle to detect an air/fuel ratio of exhaust gas exhausted from an internal combustion engine of the vehicle. The exhaust gas sensor readings may be used to control operation of the internal combustion engine to propel the vehicle.
Degradation of an exhaust gas sensor may cause engine control degradation that may result in increased emissions and/or reduced vehicle drivability. Accordingly, accurate determination of exhaust gas sensor degradation may reduce the likelihood of engine control based on readings from a degraded exhaust gas sensor. In particular, an exhaust gas sensor may exhibit six discreet types of degradation behavior. The degradation behavior types may be categorized as asymmetric type degradation (e.g., rich-to-lean asymmetric delay, lean-to-rich asymmetric delay, rich-to-lean asymmetric slow response, lean-to-rich asymmetric slow response) that affects only lean-to-rich or rich-to-lean exhaust gas sensor response rates, or symmetric type degradation (e.g., symmetric delay, symmetric slow response) that affects both lean-to-rich and rich-to-lean exhaust gas sensor response rates. The delay type degradation behaviors may be associated with the initial reaction of the exhaust gas sensor to a change in exhaust gas composition and the slow response type degradation behaviors may be associated with a duration after an initial exhaust gas sensor response to transition from a rich-to-lean or lean-to-rich exhaust gas sensor output.
The inventors herein have recognized the above issues and identified an approach for determining exhaust gas sensor degradation from an exhaust gas sensor response duration parameter based on recognition of any one of the six degradation behaviors types described above. In one example, the approach may include an embodiment of a method for determining degradation of an exhaust gas sensor positioned in an exhaust system for an internal combustion engine of a vehicle is provided. The method may include modulating an air/fuel ratio of gas exhausted by the internal combustion engine to the exhaust system through a cycle that includes at least one rich-to-lean transition and at least one lean-to-rich transition, indicating a degradation condition of the exhaust gas sensor in response to at least one of a rich-to-lean response duration of the exhaust gas sensor and a lean-to-rich response duration of the exhaust gas sensor being greater than a delay threshold, and indicating the degradation condition of the exhaust gas sensor in response to a ratio of the rich-to-lean response duration to the lean-to-rich response duration being greater than an asymmetry threshold that is relative to an axis of symmetry of the rich-to-lean response duration versus the lean-to-rich response duration.
By determining degradation of an exhaust gas sensor due to one or more of the six discreet degradation behavior types, the accuracy of exhaust gas sensor degradation determination may be improved. Moreover, by using a single response duration parameter to act as a pass/fail metric for determining degradation of the exhaust gas sensor, exhaust gas sensor degradation monitoring may be performed in a simple manner. In this way, engine control may be adjusted responsive to indication of degradation of an exhaust gas sensor to reduce the impact on vehicle drivability and/or emissions due to exhaust gas sensor degradation.